1. Field of the Invention
The present invention relates to a liquid crystal display device using a light emitting diode, and more particularly, a liquid crystal display device using a light emitting diode as a light source in the liquid crystal display device.
2. Description of the Related Art
Generally, a liquid crystal display (hereinafter, referred to as LCD) displays a desired picture on a screen by controlling the transmittance of the light supplied from a backlight unit by using a liquid crystal display panel including a plurality of liquid crystal cells arranged in a matrix and a plurality of control switches to switch video signals to be supplied to each of the liquid crystal cells. The trend in back light technology is toward miniaturization, thin profile and lightweight. In accordance with this trend, a light emitting diode (LED), which has the advantages of low power consumption, light weight and high luminance, has been suggested for use as a back light unit instead of a fluorescent lamp.
FIG. 1 is a perspective view of a direct back light unit using a light emitting diode according to a related art. Referring to FIG. 1, a direct back light unit using a LED in a LCD according to a related art comprises a plurality of LED packages 14 generating light, a printed circuit board (PCB) 12 in which a plurality of the LED package 14 is mounted at an identical interval and a diffusion plate 16 diffusing the light emitted from the LED packages 14. FIG. 2 is a perspective view of a light emitting diode package shown in FIG. 1.
As shown in FIG. 2, each of the LED packages 14 includes a LED chip 22 for generating light, a radiation plate 20, which is formed on the front surface of the LED chip 22 to reflect the light generated from the LED chip 22 outward, and a mold material for packaging the LED chip 22 and the radiation plate 20. The mold material (not shown) surrounds the LED chip 22 and the radiation plate 20 to protect the LED chip 22 and the radiation plate 20. The LED chip 22 generates red light, green light and blue light, or white light as a point light source. Such a LED chip 22 has a red LED, a green LED and a blue LED to generate the red light, green light and blue light, or white light. The radiation plate 20 formed on the front surface of the LED chip 22 has a circular shape. The circular-shaped radiation plate 20 reflect lights emitted by the LED chip with a consistent set angle ranges with respect to all axes of the circular-shaped radiation plate 20.
The PCB 12 is made of metal to disperse heat generated when driving a plurality of the LED chips 22. A controller (not shown) for controlling luminescence of the LED chips 22 is mounted on the PCB 12. The PCB 12 supports the LED chips 22.
The diffusion plate 16 is spaced at a designated interval from the LED package 14 so that the light radiated from the LED package 14 through the diffusion plate 16 onto a liquid crystal panel (not shown) has a uniform distribution. Thus, the diffusion plate 16 directs the light radiated from the LED package 14 toward a liquid crystal panel (not shown) and causes a wide angle range of light incident onto the liquid crystal panel. The diffusion plate 16 includes a transparent resin film whose sides are coated with light-diffusion materials.
FIG. 3A is a front view of the light emitting diode package viewed from a direction A shown in FIG. 2. FIG. 3B is a side view of the light emitting diode package viewed from a direction B shown in FIG. 2. As described above, a direct back light unit using the LED in the LCD according to the related art radiates the light, which radiates from the LED packages 14 mounted on a plurality of the PCBs 12, to a rear surface of the liquid crystal panel (not shown). However, as shown in FIGS. 3A and 3B, the radiation angle ranges θ1 and θ2 radiated from the LED package 14 have a maximum angle range of 120°. The radiation angle range θ1 at the front surface of the LED package 14 is the same as the radiation angle θ2 at a side surface of the LED package 14. This is because the radiation plate 20 has a circular shape. Accordingly, as the light generated from the LED chip 22 passes through the radiation plate 20, the radiation angle ranges in both dimensions are the same.
FIG. 4A is a side view of the irradiated area on the diffusion plate due to the light radiated from the light emitting diode package shown in FIG. 1. FIG. 4B is a plan view of the irradiated area on the diffusion plate due to the light radiated from the light emitting diode package shown in FIG. 1. As shown in FIGS. 4A and 4B, a radius (R) of an irradiated area 18 due to the radiated light from the LED package 14 can be determined using EQUATION 1 below.A radius (R) of a irradiated size=H×Tan(θ/2)   [EQUATION 1]In EQUATION 1, H is a height between the LED chip 22 and the diffusion plate 16 and θ is a radiation angle of light passing through the radiation plate 20. Therefore, the irradiated area size (S) can be determined using a EQUATION 2 as below.A irradiated size (S)=π×R2  [EQUATION 2]For example, if a radiation angle is 120° and H is 30 mm, then the irradiated area size 18 irradiated on the rear of the diffusion plate 16 by one of the LED package 14 is calculated as π×(30×Tan(120/2))2, which results in a value of 8432 mm2.
Accordingly, a direct back light unit using the LED in the LCD according to the related art can require a large number of the LED packages 14 depending on the size of LCD. More power is used and more is generated as the number of LED packages increases. Thus, there is a need to reduce the number of the LED packages 14 used for a direct back light unit in a LCD.